This invention has its most important application in miniature electrical cartridge fuses of the type having a centered fuse filament extending through a cylindrical, insulating housing closed by terminal-forming end caps. Some aspects of the invention, however, are applicable to fuses having a fuse filament extending diagonally across the housing.
Among the generally desired objectives in the design and manufacture of cartridge fuses are suspending a fuse filament within a desired range of tensions, soldering the same between opposed end caps in a reliable and efficient manner, minimizing the physical size of the fuse for a certain electrical rating, and maximizing protection against explosion, which can occur when the fuse filament opens or "blows" during high current, high voltage short circuit conditions that produce high energy arcs across the separated fuse filament parts. Such an arc, if unchecked, can extend to the end caps and cause an explosion. These objectives, however, are not always compatible. For example, an arc occurring in a physically small fuse can more readily spread to the end caps and create an explosion hazard than in a physically larger fuse.
One of the most common and simplest, but least reliable, ways to make a miniature cylindrical cartridge fuse is to position the fuse filament diagonally disposed across the length of the cylindrical housing and captively secure the ends of the fuse filament between the housing and the end caps. The physical and electrical attachment of the fuse filament to the end caps and of the end caps to the housing are obtained by melting a solder pellet placed in each end cap. The solder is generally drawn by capillary action into the small clearance space between the end caps and housing to anchor and seal the end caps while electrically connecting the fuse element ends to the end caps.
The blowout characteristics of such fuses having angled filaments tend to vary an undesired degree from fuse to fuse because, as the operating currents slowly rise to the melting temperature of the fuse filament, the resulting expansion or relaxation of the angled fuse filament can cause portions thereof, which are near but spaced from the housing walls, to sag and touch portions of the walls of the housing. This touching modifies the desired opening or blowing characteristics of the fuse due to the heat sinking effect of the housing walls. The degree to which each fuse filament sags and the degree to which the fuse blowing characteristics of each fuse are modified can vary substantially from fuse to fuse. Further, in the manufacturing process a delicate balance must be struck between applying enough tension to the fuse filament during the attachment process, to minimize sag, without over-stressing the fuse filament, which can produce undesired weakening of the fuse filament or the stretching thereof and also modifies the fuse blowing characteristics.
There are circumstances, however, when the fuse filament desirably extends diagonally across the housing. One such circumstance is dictated by market demand for a familiar product. Another is where a manufacturer does not possess equipment and know-how to manufacture axial mounted fuse filaments. In this last situation, the possible sagging is simply accepted or where an arc-quenching, filler material is desirable it is packed inside the housing and around the fuse filament to support the filament and prevent sagging thereof.
A better approach for making miniature fuses is to use a design with a fuse filament centered in the fuse housing. Such a disposition of the fuse filament, however, while inherently more reliable than a diagonally extending fuse filament, is more difficult to assemble and, as in the case of fuses with diagonal fuse filaments, the tension in the fuse filaments thereof is not readily closely controlled. In one process of fuse assembly, the fuse housing has end caps with centered holes therein, and a fuse filament carried by an insertion pin is passed through the fuse housing and the centered openings of the end caps. An operator then solders the fuse filament to the outer surfaces of the end caps with the fuse filament under manually applied tension. Since it is difficult for a person accurately to control the degree of tension applied to the fuse filament, in some cases inadequate tension and in the other cases excessive tension was applied which either broke the filament or unduly stretched the same, so as to undesirably modify the fuse blowing characteristics thereof. A cartridge fuse thus should be arranged easily to select the tension for the fuse filament.
Means for quenching an arc in a cartridge fuse is known. U.S. Pat. No. 3,179,773 to Keeley, discloses the use of a single arc-quenching plug formed around the fuse filament co-axially supporting only one end portion of a fuse filament extending centrally through a fuse housing. The plug prevents the arc from following the burned filament into the plug body. The plug serves no tension control function, and the other end of the fuse filament simply exits the housing end cap. When an arc reaches the point where the plug surrounds the fuse filament the material of the plug acts as an arc quenching medium but does not collapse around the burning fuse filament to quench the arc. Also, it has heretofore been routinely believed that the use of a single plug would be an adequate arc-quenching, preventive means, since the interruption of an arc at any one point interrupts the entire arc. However, the arc must reach the plug before it reaches the end cap to be effective.
U.S. Pat. No. 3,227,844 to Burrage et. al. discloses a fuse with a fuse filament passing centrally through the fuse housing and loosely through passages of a pair of insulating plugs disposed at the ends of the fuse housing so that the plugs perform no tension control or arc-quenching function since the plugs do not collapse upon the fuse filament. The fuse filament is bent over the insulating end plugs to be secured to subsequently inserted end caps by soldering, spot welding or the like. A separate filler material provides arc-quenching characteristics. The walls of the passages through the end plugs are shown spaced from the fuse filament so that, apparently, the insulating end plugs cannot hold the fuse filament in tension.
U.S. Pat. No. 4,158,187 to Perreault discloses a fuse having a diagonal or angled fuse filament. Filler material is packed inside the housing around the length of the fuse filament to quench any arc and support the filament against sagging. The end caps are attached to the housing by annular portions deformed into annular grooves in the fuse housing. The end caps are biased to a substantially fixed position relative to the annular grooves by special, resilient disk-like members compressed between the rims of the housing and the inner surfaces of the caps. The disk-like members thus provide a longitudinal force between the housing and end caps to maintain the end caps in position and reduce the depth of deformation required to attach the end caps to the housing.
The disk-like members are completely external of the housing and although there is some compression of the ends of the fuse filament between the disk-like members and the end rims of the housing, the disk-like members apparently provide no tension support of the fuse filament. This occurs because in the manufacturing procedure the ends of the fuse filament must be fixed in folded position over the rims of the housing and are free standing before the disk-like members and end caps can be fixed in place. Moreover, the two compression forces of the disk-like members against the fuse filament ends can have no effective arc-quenching action because they press against the fuse filament only for a very short length thereof and the compression points are located at the very location of the end caps from which the arc must be isolated.